Salmon Farms Spread Deadly Lice to Wild Salmon

Wild salmon stocks are being harmed by parasites from farmed salmon.

Salmon farms can pass fatal infections of sea lice to young salmon in the wild, according to a study published October 4 in the Proceedings of the National Academy of Sciences. The research, conducted in British Columbia’s Broughton Archipelago, provides the most direct evidence yet that the increase in salmon farming results in more wild salmon deaths. “Before, we knew there were potential problems,” says Martin Krkosek with the University of Alberta at Edmunton, and the lead author of the study. “Now it is very clear we have severe problems.”

According to the report, sea lice that originate in the crowded conditions of fish farms can cause mortality of up to 95 percent in baby wild salmon populations. As few as one louse can attach to a juvenile, 2.5-centimeter salmon and kill it. While adult salmon carry the parasitic lice, they are able to survive because of their size and protective scales. Moreover, in natural conditions, adult salmon typically live off shore, away from the migratory routes of younger salmon.

Salmon farms, in contrast, house hundreds of thousands of adult fish in floating pens that are anchored within natural migratory channels, a setup that causes baby wild salmon to encounter clouds of sea lice as they swim by. The study compared three different salmon migration routes, netting 17,000 fish at regular intervals for two years. Mortality rates ranged from 9 to 95 percent, with the highest fatalities occurring in channels with the most salmon farms, during the end of the migration season when sea lice are most prevalent.

Ransom Myers, professor of biology at Dalhousie University in Nova Scotia, has called the study the most comprehensive to date on the issue, and hopes it will move the Canadian government to protect wild salmon populations from rapidly growing fish farms. But fish farming business is a lucrative business. Canada’s roughly 280 salmon farms produce 96,000 tons of fish annually, bringing in US$387 million. (The United States, in contrast, is home to only nine salmon farms.) Worldwide, aquaculture is growing by 10 percent annually as ocean fisheries are increasingly depleted, according to the study.

This story was produced by Eye on Earth, a joint project of the Worldwatch Institute and the blue moon fund. View the complete archive of Eye on Earth stories, or contact Staff Writer Alana Herro at aherro [AT] worldwatch [DOT] org with your questions, comments, and story ideas.